1. Field of the Invention
The present invention relates to a large thread groove type vacuum pump having a large flow rate and a large pumping speed used for a vacuum pumping system, for example, of a nuclear fusion installation.
2. Description of Background Art
The pumping performance represented by the compression ratio and the pumping speed substantially depends on a gap between a rotor and a stator. It is known that the smaller the gap, the higher the pumping performance. Under a medium vacuum condition such as several tens Pa, a smaller gap is required for lighter gas in order to maintain the desired pumping performance.
Especially in a vacuum pumping system of a nuclear fusion installation, the gap should be made as small as possible since a large amount of light gases such as hydrogen, hydrogen isotope and helium have to be continuously discharged at a maximum level of several tens Pa during the plasma combustion. Since a problem of rotor/stator contact would be caused if the gap is too small, attempts have been made to keep the gap constant along the gas flow passage between the rotor and the stator so that it operates safely.
A method is also known to previously estimate the expansion of the outer diameter of the rotor caused by centrifugal force generated by the rotation of the rotor and then to determine the inner diameter of the stator corresponding to the estimated expansion (e.g. Japanese Laid-open Utility Model Publication No. 91096/1989).
In FIG. 5 which is a longitudinal cross-sectional view showing a thread groove type vacuum pump of the prior art, reference characters "A", "B" and "Cr" denote a rotor, a stator, and a gap between the rotor and the stator, respectively.
The gap between the rotor and the stator should be determined based on the expansion of the outer diameter of the rotor due to its thermal expansion rather than that due to centrifugal force. According to the conventional design, the gap has been determined in accordance with the maximum temperature rise of the rotor in order to avoid the problem of rotor/stator contact.
In the thread groove type vacuum pump used in the vacuum pumping system of a nuclear fusion installation, the inner diameter of the stator is determined to ensure safe operation during the initial air exhaustion since the rotor temperature reaches a maximum during the initial air exhaustion.
On the other hand, since the primary object of the thread groove type vacuum pump used in the nuclear fusion installation is the exhaustion of plasma combustion gases including mainly hydrogen, hydrogen isotope and helium which are lighter than air, the temperature rise of the rotor in this case is less than in the case of exhausting heavier gases such as air. Accordingly, the gap determined in accordance with the conventional design becomes unnecessarily large for use in a nuclear fusion installation. Thus, the design of the prior art extremely lowers the pumping performance of the thread groove type vacuum pump used in the exhaustion of plasma combustion gas and requires an increase in the number of the thread groove type vacuum pumps installed in order to obtain a desired pumping speed.
Since the installation space for the vacuum pumps is limited, a large thread groove type vacuum pump having superior pumping speed per pump is required. However, the change in the gap due to the temperature change must be increased since a large diameter rotor is used in the large pump.
Examples of how the thread groove type vacuum pump of the prior art is affected by different gaps are shown in FIGS. 3 and 4 wherein a performance curve of a large thread groove type vacuum pump having a gap Cr of 0.5 mm used for hydrogen gas is shown in FIG. 3 and that having a gap Cr of 0.78 mm is shown in FIG. 4.
The thread groove type vacuum pump used in these examples has a rotor having a diameter of 600 mm and a length of 800 mm as well as a design specification of a suction pressure Ps of 10 Pa and a flow rate Q of 10.sup.4 Pa.multidot.L/s at a revolution of 142 rps. As can be seen from FIGS. 3 and 4, although it is possible to keep the suction pressure Ps constant within the exhaust pressure of about 200 Pa when the gap Cr is 0.5 mm, the suction pressure Ps is drastically increased when the exhaust pressure exceeds 100 Pa. Accordingly, an auxiliary pump arranged after the thread groove type vacuum pump having the gap Cr of 0.78 mm is required to have a pumping speed twice that of an auxiliary pump arranged after the thread groove type vacuum pump having the gap Cr of 0.5 mm.
Alternatively, when using the same auxiliary pump in both cases, if the exhaust pressure of the thread groove type vacuum pump is set at 200 Pa at a flow rate Q of 10.sup.4 Pa.multidot.L/s, the suction pressure Ps would become 10 Pa when the gap Cr is 0.5 mm and would be increased to 20 Pa when the gap Cr is 0.78 mm.
In addition, as hereinafter described in a second embodiment, when the gap Cr is changed from 0.5 mm to 0.78 mm in order to correspond to the temperature difference 40.degree. C. in the temperature rise of the rotor, it has been found in the conventional thread groove type vacuum pump that the pumping speed is reduced by half.
Thus, in the thread groove type vacuum pumping system used in a nuclear fusion installation, it is necessary to have a structure in which the gap between the rotor and the stator is not influenced by a temperature change of the rotor.